The present invention relates to an improved fired, fireclay-bonded high alumina refractory and to its method of preparation; and, more particularly, the invention relates to a fired, fireclay-bonded high alumina refractory having improved properties, especially resistance to erosion by molten slags and metals while being substantially free from black core, to its method of preparation and to the green, pre-fired body from which it is made.
The manufacture of high alumina refractories, like brick, that is, refractories containing over 50% Al.sub.2 O.sub.3, generally involves blending various high and low alumina refractory materials. Traditionally, clays with less than 50% Al.sub.2 O.sub.3 are enriched with a high alumina material such as calcined bauxite; calcined or tabular alumina; kyanite; raw or calcined fireclay, and the like. Other non-basic refractory aggregates well known in the refractories field may be included. Carbonaceous material, principally tar, pitch or graphite, is incorporated in the refractory generally by impregnation after firing in the case of tar and pitch; however, particulate carbon, such as graphite or milled pitch, may be added to the mix before shaping and firing.
Impregnation of high alumina brick with hot tar or pitch must be completed after the brick has been fired to prevent oxidation of the tar or pitch. This requires additional handling, time and cost. Moreover, with tar-or pitch-impregnated brick used as a lining for vessels handling "hot metal", as in torpedo cars and iron handling linings, a large amount of fume is generated when the volatiles burn out. This fume represents a health and environmental hazard.
Another problem associated with fired, fireclay-bonded high alumina refractory brick is what is known as "black core". Black core brick, aside from aesthetic aversion by masons to the discoloration, possess other properties which would be undesirable especially in hot metal car applications. Such undesirable properties include reduced thermal shock and spalling resistance.
Black core is the result of incomplete oxidation of carbonaceous matter in the raw materials, and is caused by iron being held in the reduced state. This reduced state is brought about by carbon impregnating the microstructure as carbon monoxide. If the iron in the mixture is still in the ferrous (reduced) state at a temperature above 1900.degree. F., it will behave as a flux. This behavior further seals the microstructure, inhibiting the flow of oxygen and the products of combustion, ultimately resulting in black core.
It has been suggested to eliminate carbonaceous material from high alumina refractories and to include very finely divided SiC as a source of carbon. However, such simple substitution as suggested by the prior art does not eliminate black core.
McCreight et al. U.S. Pat. No. 3,164,483 discloses a high alumina refractory made without carbonaceous material, such as tar, pitch or graphite, and containing very finely divided SiC; that is, containing at least 5%, by weight, -325 mesh SiC and preferably more. Takeda et al. U.S. Pat. No. 3,892,584 discloses monolithic high alumina refractory materials containing SiC and carbon. The product is fired in a reducing atmosphere. Cherry U.S. Pat. No. 4,093,470 discloses a high alumina refractory containing the usual graphite and a volatilized silica binder. An optional addition is SiC.
It is the principal object of the present invention to provide a fired, fireclay-bonded high alumina refractory possessing improved properties including especially substantial freedom from black core and, hence, attendant characteristics associated with that condition.
Another object of the present invention is to provide a fired, fireclay-bonded high alumina refractory substantially devoid of elemental carbon, as from tar, pitch, graphite and the like, and containing as substantially the only carbon-containing material a metallic carbide of critically-selected particle size distribution to provide a product possessing improved properties including essential absence of black core.
These and other objects including the provision of an unfired, mix adapted for firing under oxidizing or reducing conditions to provide the improved fired refractory as well as methods for preparing the unfired and fired products will become apparent from a consideration of the following specification and claims.
It has been found that, in the preparation of fired, fireclay bonded high alumina refractories especially adapted for forming into shapes, like brick, and firing under either oxidizing or reducing conditions, the substantial elimination of carbon, as from carbonaceous materials such as tar, pitch, graphite, and the like and the inclusion of a particulate metallic carbide in a defined amount and of a critically selected particle size distribution, provides a refractory product possessing improved properties as discussed more in detail hereinafter.